Method of refining the macrostructure of titanium alloys



United States Patent 3,470,034 METHOD OF REFINING THE MACROSTRUCTURE 0F TITANIUM ALLOYS Joseph Kastanek, Youngstown, and Harold D. Kessler, Canfield, Ohio, assignors to Reactive Metals, Inc., a corporation of Delaware No Drawing. Filed Feb. 14, 1967, Ser. No. 615,928

Int. Cl. C22f N00 US. Cl. 148-11.5 Claims ABSTRACT OF THE DISCLOSURE A process is disclosed for producing a fine-grained titanium alloy macrostructure which involves heating an ingot or billet to a temperature between 50 and 250 F. above the alloys beta transus and hot working, for example by forging, the heated alloy as its temperature decreases to within the range of 50 to 300 F. below the beta transus. The performing of additional cycles of the aforementioned sequence of heating and hot working results in progressively finer grain size.

This invention relates to a process for producing a fine-grained titanium alloy macrostructure. More particularly, the invention involves a method of processing titanium alloy billets or ingots by heating and forging to produce a fine-grained structure susceptible to very exacting ultrasonic inspection.

The macrostructures of ingots or billets of alpha-beta and beta type titanium alloys typically show traces of former beta grains referred to in the art as ghost grains. Ingots and billets containing ghost grains have proved to be diflicult to inspect to very exacting ultrasonic standards because of excessive interference of the normal reflection of the sound waves. The relatively large former beta grains which interfere with ultrasonic testing result from the general practice in the titanium industry of hot working, eg by forging, material at lowest possible temperature below the beta transus in order to produce a fine grain product. In small forgings where thicknesses do not exceed 3 to 4 inches, this lowtemperature forging practice produces satisfactory results. However, in heavier billets or forgings, the center section is generally not as well refined and large former beta grains may persist.

Metallurgical practices designed to produce sonically quiet billets, i.e. billets with minimum interference to the sonic testing, have included press or hammer forging of the billets with various reductions from temperatures at least 100 below the beta transus temperature. We have found that working the metal at temperatures well below the beta temperature refines the micro grain but does not refine the macro grain structure, particularly at the center. The present invention provides a process for producing a fine-grained titanium alloy macrostructure throughout the work which allows the material to be ultrasonically tested to very exacting standards.

Ultrasonic testing of ingots and billets is performed with the use of a transducer which creates vibrations directed to the surface of the material, eg, billet, to be tested, the vibrations impressed on the billet travel through the billet and are reflected from the back surface. If defects are present, the vibrations are deflected and the deflections point up the location and nature of the defects. Coarse grains within the billet also deflect the sonic waves and thereby interfere with the sensitive pick-up of the waves deflected by deflects. The interference by the coarse grains is sometimes referred to in the industry as background noise. Standards are used in 3,470,034 Patented Sept. 30, 1969 ultrasonic testing which compare the observed defects with a standard simulated defect. Simulated defects are created by a so-called flat bottom hole (FBH) of a given depth and controlled diameter formed in a specimen of the same material undergoing test. The severity of the tests is primarily controlled by the diameter of the hole of the standard. The hole diameters are measured in 64ths of an inch and smaller diameter specimens represent testing to a higher standard. FBH standards of and %4-lI1Ch represent a high level of inspection in billets 8 to 12 inches in diameter and are commonly used. According to the terminology employed, a simulated defect of a standard hole of i -inch diameter is referred to as a No. 3 FBH standard. A -inch diameter hole is referred to as a No. 5 FBH standard. Minimizing or precluding the presence of coarse grains in the ingot or billet improves the ability of the material to be tested to more exacting ultrasonic testing standards by reducing the interference of the macrograins with deflections by defects contained in the billet.

The present invention provides a method for producing a fine-grained titanium alloy macrostructure which comprises heating the titanium alloy billet or ingot to a temperature between 50 and 250 F. above the alloys beta transus and then hot working, for example by forging, the heated alloy as its temperature decreases to within the range of 50 to 300 F. below the alloys beta transus. The term beta transus refers to that temperature at which the body-centercd-cubic beta phase transforms to hexagonal-close-packed alpha phase microstructure. The beta phase exits from the beta tranus temperature to the melting point of the material. It may also be retained at temperatures below the beta transus by alloying and/or heat treating.

In accordance with a preferred embodiment of the invention, the aforementioned sequence of heating and hot working is performed more than once. The degree of refinement of the macrostructure is influenced by the number of cycles (heating and forging) to which the material is subjected. While for some applications one cycle is sufficient, several cycles are normally preferred. In each cycle, the amount of hot working reduction is advantageously from 25 to (total reduction). It is also desirable in some applications to restrict the reheating range at the final reheating to within of the beta transus.

The following example will illustrate a preferred practice of the invention. A 30-incl1 diameter ingot of a titanium alloy containing 6% Al and 4% V having a beta transus temperature of about 1825 F. is heated to 2100 F. After heating, the ingot is pressed to a 24-inch square in a forging press. The temperature during press forgings decreases to about 1650 F. after which it is reheated to 2100 F. Following reheating, the forged ingot is pressed to a 17.5-inch square configuration. At this point, it may be cut into smaller sections for handling and then reheated to 2100 F. prior to a subsequent press forging to a 14-inch square cross section. In each of these cycles of heating and working, the ingot is recrystallized to achieve a decrease in grain size during the subsequent working.

In this example, following the pressing to 14-inch cross section, the billet is reheated to 2100 F. and then press forged again to a cross section of 11 x 13 inches. At this point, the long ingot is cut to section lengths as desired which are each conditioned, preferably by surface grinding. As a final reheating step, the ingot sections or billets are reheated to 1865 F. and pressed to a l0-inch square. If further reductions are necessary, the billets may be reheated as many times as desired and press forged. Tests of the billet section may be taken from time to time for various chemical and physical determinations.

Ultrasonic testing of the billets treated as described showed no indication of ghost grain interference and photomacrographs show a fine equiaxed macrostructure, which is easily penetrated by the ultrasonic sound waves, and the background noise and false indications associated with variable grain size are virtually eleminated.

The aforementioned billet, when forged as described and machined to a 9-inch diameter finish, may be sonic tested using a -inch diameter flat bottom hole as a standard without indications in excess of the sonic standard due to the presence of coarse former beta grains.

It is apparent from the above that various changes may be made without departing from the invention. Thus, for example, various forms of hot working may be used, such as rolling and/or either hammer or press forging may be practiced to obtain the desirable fine-grained structure. This structure will possess improved physical properties as well as improved ability for ultrasonic inspection. Hot Working at sufiicient high temperatures in the beta phase causes continuous recrystallization. By repeated cycles of hot or warm working as the material cools below the beta transus with sufficient reductions, recrystallization to produce progressively finer grain of the beta phase occurs on subsequent reheating above the critical temperature.

We claim:

1. A process for producing a fine-grained titanium alloy macrostructure comprising subjecting a titanium alloy billet to at least two cycles of heating and working comprising heating to a temperature between 50 and 250 F. above the alloys beta transus and hot working said heated alloy as its temperature decreases to within the range of from 50 to 300 F. below the alloys beta transus.

2. A process according to claim 1 wherein said alloy is hot worked by forging.

3. A process according to claim 1 wherein said sequence of heating and hot working is performed at least three times with reductions during forging of from 25 to 90% each time.

4. A process according to claim 3 wherein prior to the last reduction the alloy is heated to within 100 F. above its beta transus.

5. A process according to claim 4 wherein the alloy is heated to within 50 F. above its beta transus.

References Cited UNITED STATES PATENTS 3,169,085 2/1965 Newman 148l1.5 3,394,036 7/1968 Parris 14811.5

L. DEWAYNE RUTLEDGE, Primary Examiner W. W. STALLARD, Assistant Examiner 

